The invention relates to a manually operable hydraulic pilot-control device.
DE 27 51 946 C2, for example, discloses a manually operable hydraulic pilot-control device. It works on the basis of continuously adjustable pressure-reducing valves, at the control outlet of which a pilot-control pressure dependent upon the deflection of a hand lever can be set. The hydraulic pilot-control device shown in the above-mentioned specification has, in a housing, a continuous housing bore, which, in the interior of the housing, has a control section, in which the control piston of a pressure-reducing valve is guided tightly in an axially displaceable manner. Bore sections of larger diameter follow on both sides of the control section. Screwed into one end of the housing bore is a guide bush, in which a plunger is guided in an axially displaceable manner, which plunger projects away from the housing beyond the guide bush and can run with a collar on its inner end against the guide bush. At its end projecting outward beyond the guide bush, the plunger can be acted upon by a hand lever via an intermediate member. On the inside, a spring retainer is pressed against the plunger by two helical compression springs. One helical compression spring is a restoring spring for the plunger and for the hand lever and is supported on a shoulder of the housing bore in addition to being supported on the spring retainer, this shoulder limiting the control section of the housing bore in the one direction, The second compression spring is the control spring of the pressure-reducing valve, and this control spring is supported on a shoulder of the control piston in addition to being supported on the spring retainer and can thus act upon the control piston in a direction away from the plunger. The control piston extends with a long neck through the control spring and, by means of a head, engages behind the spring retainer at the plunger. When the head bears against the spring retainer, there is a clearance space between the head and the plunger in the axial direction, and this clearance space permits a relative displacement between control piston and plunger.
The control piston is a hollow piston having an axial blind bore, which, at the control-piston end face remote from the plunger, is open toward a housing-bore section which is designed as a screw connection and constitutes the control outlet of the pressure-reducing valve. Running between the blind bore and the outside of the control piston are a plurality of radial bores, via which, as a function of the position of the control piston, the space in which the restoring spring and the control spring are located and which is connected to a tank outlet of the pilot-control device, or a control space which is connected to the pressure inlet of the pilot-control device, can be connected to the blind bore and thus to the control outlet. The spring space, containing the restoring spring and the control spring, and the control space are separated from one another by a narrow housing web, the axial extent of which is slightly greater than the diameter of the radial bores in the control piston. The control piston is located in the control position when the radial bores are covered at least almost completely by the housing web. In this control position there is equilibrium between the axially directed forces acting upon the control piston. The force acting from the control spring in the one direction is just as large as the opposing force produced by the pressure at the control outlet on the cross-sectional area of the control piston. Small movements of the control piston out of the control position connect the blind bore and thus the control outlet to the tank outlet or the pressure inlet, as a result of which the pressure in the control outlet is kept largely constant in a certain position of the plunger.
In the zero position of the hand lever and the plunger, the radial bores in the control piston are at a distance from the control space connected to the pressure inlet. The control spring is restrained between the control piston and the plunger with a certain preloading force.
With the known pilot-control device, a pilot-control pressure according to the control curve shown in FIG. 2 can be set as a function of the deflection of the hand lever or as a function of the stroke of the plunger. At the start of a movement of the hand lever out of the zero position, the control piston is driven along by the plunger via the control spring and the spring retainer at the plunger, without a pressure first building up in the control outlet. This idle travel is determined by the initial distance between the radial bores in the control piston and the control space connected to the pressure inlet of the pilot-control device. As soon as a cross section of flow is opened between the radial bores and the control space, the control pressure jumps to a value which is determined by the preloading force of the control spring in the zero position. Upon further deflection of the hand lever and further displacement of the plunger, the control spring is compressed further, while the control piston remains in the region of its control position or returns into its control position after the build-up of the corresponding pressure in the control outlet. In accordance with the linear characteristic of the control spring, the control pressure increases linearly with the plunger stroke.
In some applications it is desired that the idle travel and the extent of the initial jump in the pilot-control pressure correspond to very accurately determined values. This is the case, for example, in vehicles which are equipped with separate hydrostatic drives for the two sides of the vehicle, for example for two crawler tracks, and are steered by two drive wheels on opposite sides of the vehicle being driven at different speeds. The idle travel at the pilot-control device ensures that small movements of the hand lever on the pilot-control device, which are caused by vibrations for example, do not already lead to a different speed of the drive wheels and thus to a steering deflection. On the other hand, idle travel at the pilot-control device means steering play, which is to be limited to a certain value. Both factors together lead to the demand that the idle travel is to be maintained very accurately.
It is also desirable that the extent of the pressure jump at the end of the idle travel and a response threshold of the hydraulic main device which is activated by the pilot-control device, for example the response threshold of an operating cylinder for the adjustment of a variable displacement pump, are matched to one another. If the pressure jump at the pilot-control device is lower than the threshold at the main device, the lever or plunger travel at the pilot-control device increases until the main device responds. This makes itself felt in an increase in the idle travel and, in the case of steering controlled at different speeds, in increased steering play. If the initial jump in the pressure is higher than the response threshold of the main device, the pilot control becomes less sensitive.